SUPERFICIAL DETRITUS. 
319 
All the northern part of the district, and the low slopes and deeper valleys of the southern 
part, are covered to a greater or less depth by superficial materials of more northern origin, 
mingled with those of the rock on which the deposit rests. 
The extreme northern margin consists of the worn fragments of lower stratified rocks, as 
the Hudson river group, intermingled with a large proportion of those of the Medina sand¬ 
stone. The former gradually diminish as we progress southward, and are finally lost alto¬ 
gether ; those of the different successive formations taking their place, and constituting, in 
turn, either the greater or smaller part of the whole accumulation. Those which are of the 
most durable character continue farthest, and they may even be traced as far as the Penn¬ 
sylvania line; but, in their passage southward, they have been much worn, and greatly dimi¬ 
nished in size. Still, by careful examination, traces of all the stratified rocks on the north 
may be met with in these depositions. 
As we pass southward, however, over the successive formations, we find that all have suf¬ 
fered greatly from denudation, and that the abraded fragments of each constititute a large 
proportion of the superficial materials resting on its southern neighbor. The size of the 
fragments always bears a proportion to the distance they have been transported from the 
parent rock; and in the Fourth District we often find a huge mass of a northern rock resting 
upon the margin of the one next south of it, while at a distance of ten or twenty miles farther 
south, only small pebbles of the same occur. 
These remarks apply to the older deposits ; and while it sometimes happens that a huge 
fragment has been transported many miles southward, having suffered little attrition, it 
seems due to some operation subsequent to that of the great accumulation below. Even in 
the deep valleys at the southern margin of the State, may be found pebbles of all the northern 
rocks of the district, except the soft shales, which will not withstand the transportation. 
We find resting upon the limestone and calcareous shale formations a dark colored gravel, 
ranging from fine to coarse. This has accumulated most in sheltered situations where a 
succeeding bed or stratum rises to the south of it, as if it had been pushed onward over the 
bottom, lodging against the projecting edges of the strata. In this way it often happens that 
a rock, in its more northern extension, is covered with sand or clay, while the coarser mate¬ 
rials are pushed farther onward. These phenomena occur where the deposit is evenly distri¬ 
buted over the surface: in other places the coarser and finer materials, intermingled in the 
greatest confusion, are heaped up into conical hills, which are thickly scattered over the sur¬ 
face. Again, the same materials are accumulated in long hills or ridges, having a determi¬ 
nate direction, and sloping down from a high northern elevation to the general level of the 
country on the south. 
The Drift or Boulder period, the products of which are often confounded with more ancient deposits of similar materials, is cer¬ 
tainly the most recent of all, and, except where they are intermingled with the previous deposits, are always the most superficial. 
In the valley of the Hudson, in the vicinity of Albany and Troy, I have searched in vain for a boulder or pebble of granite, or of 
any rock older than the Potsdam sandstone, in the deposits below the clay ; while in a period subsequent to the deposition of the 
clays and sands, boulders of granite are by no means rare. 
It may be remarked here, that the old deposits alluded to contain abundance of quartz pebbles, and quartz with green chlorite ; 
but these are from the quartz veins in the partially altered rocks of the age of the Hudson river group. 
